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NBME 22 Answers

nbme22/Block 1/Question#50 (29.3 difficulty score)
A 9-year-old boy is brought to the physician ...
Protein structure🔍

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rcdesInea twaes dan a+N rceotaincnnot ldhuso tponi to itccsy firibsos F()C. hTe rpelmob tihw FC is tno htat eht eegn si ingbe dsbntaicrer l,ses tub htat het nroetip atth eth egen eoscd rfo is areetld, hwcih sedal ot the FC ehnacln nebig ddadereg deu ot dmsng-iliof gt;-&- less FC roercetsp on clle suafecr t&g--; opcyihntep F.C

ls3076  why not membrane receptor? +6  
a1913  delF508 is a 3 base pair deletion of phenylalanine at amino acid position 508. Mutation causes impaired post-translational processing of CFTR (improper folding) which rough ER detects. Sends mutant misfolded CFTR to the proteasome for degradation, preventing it from reaching cell surface. So problem is not malfunctioning CFTR channels in the surface; problem is complete absence of CFTR on cell surface (since they keep getting misfolded and sent to proteasome to be trashed). Source of primary problem: error in protein structure +6  
angelaq11  @Is3076 because the CFTR is a channel not a receptor. +16  
rainlad  FA 2019 p. 60 +  
dysdiadochokinesia  @a1913 is correct- as for @angelaq11, you can still have a receptor that also functions as a channel as they are not mutually exclusive. An example of this is the nAChR found on postsynaptic NMJ neurons. This is a non-selective, ligand-gated, ionotropic receptor that functions as a channel once its ligand (i.e., ACh) has bound to the active site to induce conformational change. Similarly on the same realm: CFTR is an ionotropic receptor that concurrently functions as a Cl- channel once its ligands (ie. 2 ATP) is bound to open the channel and enable Cl- flux. This question in particular is asking for the underlying pathophysiologic mechanism for cystic fibrosis, which boils down to an issue with the primary structure of a protein resulting in its misfolding and subsequent sequestration/degradation. +1  

FYI this is nth* question I've seen that wants to see if you know that the CFTR mutation results in a misfolded protein that gets stuck in the rER.

Where n = a very big number

castlblack  I knew that but I thought the chaperone proteins counted as protein regulation :( +1  

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